FUEL PUMP PROBLEMS. PLEASE REPLY ASAP!

Hi,
I have a 1988 ford f150 custom, with a 351 winsor motor. It has dual gas tanks, but as the front one did not work when I bought it, Ive been using the rear. However, the guage has not been accurate, and would move when I hit a speed bump, or would go all the way up past full when the tank was not full at all. We decided to replace the fuel pump/float assembly, so we dropped the tank and replaced it, reconnected all the hoses and wire harness correctly, and bolted the tank back up. We put some gas in it, and noticed that the guage did read fairly accurately. However, when we started it, it died after a few second repeatedly, and it appeared that the pump was not working. Could the new pump be faulty, or is there another cause that we failed to find? Any suggestions are greatly appreciated. Thanks,
-Anthony-